Elsevier

Veterinary Microbiology

Volume 179, Issues 3–4, 30 September 2015, Pages 142-148
Veterinary Microbiology

Viral load of equine herpesviruses 2 and 5 in nasal swabs of actively racing Standardbred trotters: Temporal relationship of shedding to clinical findings and poor performance

https://doi.org/10.1016/j.vetmic.2015.06.002Get rights and content

Highlights

  • 79% of 66 Standardbred trotters were co-infected at least once with EHV-2 and EHV-5.

  • 30% of the nasal swabs were positive for EHV-2 and EHV-5 was detected in 74%.

  • Highest load of EHV-2 and -5 excretion was detected in spring and summer, respectively.

  • High viral load of EHV-2/-5 was not associated with clinical respiratory disease.

Abstract

The equine gamma herpesviruses 2 and 5 (EHV-2 and -5) have frequently been observed in the equine population and until recently presumed low to nonpathogenic. However, recent reports linking presence of equine gamma herpesviruses with clinical signs of mild to severe lung disease, suggest that the role of these viruses in respiratory disease and poor performance syndrome is still unclear. Moreover, baseline data regarding the temporal pattern of shedding of EHV-2 and EHV-5 within stables and within individual actively racing horses have been lacking. In a prospective longitudinal study, we followed elite racing Standardbred trotters at monthly intervals for 13 months, to investigate whether the amount of EHV-2 and EHV-5 shedded in nasal secretions varied over time within and between individual horses. Sixty-six elite horses were investigated by analyzing nasal swabs and serum samples, a health check and evaluation of athletic performance monthly during the study period. Nasal swabs were analyzed with two newly developed qPCR assays for EHV-2 and EHV-5, respectively. Of 663 samples, 197 (30%) were positive for EHV-2 and 492 (74%) positive for EHV-5. Furthermore, 176 (27%) of the samples were positive for both EHV-2 and EHV-5 simultaneously. There was considerable variation in the amount and frequency of shedding of EHV-2 and EHV-5 within and between individual horses. Viral load varied seasonally, but neither EHV-2 nor EHV-5 viral peaks were associated with clinical respiratory disease and/or poor performance in racing Standardbred trotters.

Introduction

The closely related equine gamma herpesviruses 2 and 5 (EHV-2 and EHV-5) are poorly understood but under increasing scrutiny in the equine world. Herpesviruses are characterized by lifelong latent infection in the host, where the cells in which they remain latent vary dependent on type of herpesvirus (Roizmann et al., 1992). In humans, the gamma herpesvirus Epstein–Barr virus (EBV) is asymptomatic or causes very mild symptoms if the infection occurs in childhood, whereas infection later in life is associated with mononucleosis (Balfour et al., 2013a). Most reviews indicate that >90% of the adult population in the USA are persistently infected with EBV whereas the antibody prevalence in children is around 50% (Balfour et al., 2013b).

In horses, both gamma herpesviruses EHV-2 and EHV-5 infect individuals early in life (Murray et al., 1996, Bell et al., 2006, Wang et al., 2007). Therefore, they are commonly detected in all age groups in healthy equine populations (Kemeny and Pearson, 1970, Bell et al., 2006, Borchers et al., 2006, Torfason et al., 2008, Rushton et al., 2013) and were presumed to be largely non-pathogenic.

Recent reports have challenged these assumptions, with an increased frequency of reports linking presence of equine gamma herpesviruses with various clinical problems. For example, EHV-2 has been incriminated in mild respiratory disease (Dunowska et al., 2002, Wang et al., 2007), tracheal inflammation (Fortier et al., 2009) and keratoconjunctivitis (Kershaw et al., 2001), while EHV-5 has been linked to occurrence of equine multinodular pulmonary fibrosis (EMPF) (Williams et al., 2007). Earlier work also suggested that EHV-2 infections may be associated with poor athletic performance in race horses (Studdert, 1974). Previous studies have shown that younger horses are more likely to have a higher viral load of EHV-5 compared to adults (Marenzoni et al., 2010) and in an EMPF case viral load was higher in implicated organs, like the lung (Marenzoni et al., 2011). On the other hand, viral load of EHV-2 in a cohort study of foals followed over five months showed no relationship with occurrence of clinical signs (Brault et al., 2011). To date however, no studies have followed horses over time to evaluate the relationship between infection of equine gamma herpesvirus and poor performance. Thus, information about the pattern of viral load of gamma herpesviruses in the horse population is scanty, and the role of EHV-2 and EHV-5 in clinically detectable disease, in particular during periods of high viral shedding, is not known.

The aim of this work was to monitor qualitatively and quantitatively the excretion of EHV-2 and EHV-5 in the upper respiratory tract, as markers of viral load in elite racing Standardbred trotters to identify the dynamics of viral shedding within and between horses, and over seasons. Additionally results were used to assess whether variations in viral load could be associated with selected indices of poor performance or clinical respiratory disease. To accomplish this we also needed to develop and validate two single qPCR assays specific for EHV-2 respective EHV-5 targeting the DNA polymerase gene, as recently published methods were not available for us at the time (Rushton et al., 2013, Hue et al., 2014).

Section snippets

Description of the cohort

From August 2010 to August 2011 (13 months), a cohort of 66 actively racing Standardbred trotters was followed at monthly intervals. At the time of recruitment the horses were healthy and well performing with a mean age of 3 years, (range: 2–8 years, SD: 1.33) and kept at four different professional training yards, as previously described (Back et al., 2015). Throughout the study period, all horses trained and raced according to their normal schedule and the same veterinarian monitored their

Description of the cohort

Two of the 66 horses were excluded due to change of trainer and one horse was excluded upon request of the owner. From the remaining 63 horses a total of 663 NS samples were taken during the study period. Regarding occurrence of abnormal clinical signs at the time of sampling, 1% had fever (9/663 of which one horse had cough at the same occasion and another had nasal discharge together with fever), 1% cough (8/663) and 1% nasal discharge (6/663) (described in more detail in Back et al., 2015).

Validation of the qPCR assays

Discussion

In this longitudinal study, both EHV-2 and EHV-5 were commonly detected in nasal secretions from the horses, which is in agreement with previous studies worldwide (Wang et al., 2007, Pusterla et al., 2013, Hue et al., 2014). For EHV-5, 74% of the samples were positive on qPCR assay and 40% of the horses were uniformly tested positive throughout the study. Since a large group of horses was followed over a year, the prevalence of EHV-2 and EHV-5 presented in Table 1, Table 2 provides invaluable

Conclusions

It appears that both EHV-2 and EHV-5 are endemic in this population of racing Standardbred trotters and although there was a large variation in viral load, both within individual horses and between EHV-2 and EHV-5, and over seasons. However, neither the presence of EHV-2/-5, nor the timing of the peak of the viral load was associated with clinical respiratory signs, markers of systemic inflammation, pharyngeal or tracheal inflammation or poor performance. Since the peaks of viral load of EHV-2

Acknowledgements

The authors wish to acknowledge Dr. Nils Ronéus for scientific contribution and thank the trainers and owners without whom this research would not have been possible. This study was funded by Swedish-Norwegian Foundation for Equine Research project no H1247140, and the endoscopic equipment was provided by Scandivet, Sweden.

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